Abstract

Three rhesus monkeys were studied during daily three-hour sessions in which key presses (responses) produced i.v. infusions of 0.25 mg/kg of pentobarbital. Under one-response fixed-ratio schedule conditions (every response followed by drug infusion; FR 1), increasing the pentobarbital infusion dose produced dose-related decreases in self-administration and only small increases in total daily pentobarbital intake. Gradually increasing the number of responses required per 025 mg/kg infusion to 10 (10-response fixed-ratio schedule; FR 10) markedly decreased pentobarbital self-administration and total daily pentobarbital intake. Only at higher infusion doses, which produced periods of general anesthesia and of extremely low response rates, was the number of pentobarbital infusions similar under both the FR 1 and FR 10 schedule conditions. Three monkeys were studied under comparable conditions with key presses producing 0.05 mg/kg infusions of cocaine. Increasing the cocaine infusion dose produced dose-related decreases in self-administration and only small increases in total daily cocaine intake. Gradually increasing the FR response requirement from 1 to 10 changed neither the number of self-administered 0.05 mg/kg cocaine infusions nor total daily cocaine intake.